The production of liquefied natural gas (LNG) is carried out at cryogenic temperatures, typically at about -260 degrees Fahrenheit (-162.degree. C.). These low process temperatures are typically achieved by the use of an external refrigeration system that employs either a mixture of refrigerants in a single loop or a cascade of pure refrigerants. Either process requires a large input of compressor work to achieve the low temperature. In fact, the recognition that this power input is a requirement of all commercial LNG processes has led the industry to adopt a measure of efficiency called "specific power," defined as the energy input required per unit of LNG produced. In addition, the refrigeration equipment required to achieve LNG production temperatures is expensive to purchase and maintain. Generally, the cost of such equipment increases as specific power requirements increase. For both these reasons it is highly desirable to provide a LNG process that minimizes or at least reduces the specific power.
The typical gas feed stream to a LNG plant includes non-associated gas, i.e. gas which is not derived from oil production. This gas feed stream contains primarily methane with minor quantities of ethane, propane and butane. If, however, the gas feed stream is associated gas (i.e. gas which evolves from oil production), it is still predominately methane, but can contain significant quantities of ethane, propane, butane, and C.sub.5 and heavier hydrocarbons often referred to as "condensate."
A number of process are reported in the prior art for extracting natural gas liquids (ethane, propane, butane and condensate, known collectively as NGL) from an associated gas stream. These extraction processes typically operate by sequential distillation of the gas stream to extract the individual hydrocarbon components. The stabilized condensate is often added back to the crude oil. Propane and butane fractions are recovered separately and sold while the methane and ethane fractions are typically compressed and reinjected back into the oil reservoir to maintain the formation pressure.